1. Field of the Invention
This invention relates in general to antennas and more particularly, to mult-band, multi-function antennas.
2. Description of the Related Art
In civilian life, wireless communication has become a luxury many feel they can't live without. In military operations, that may literally be true. In the field, soldiers must be able to communicate reliably and efficiently with others on the land, in the air, sea, and on the opposite side of the world. Wireless communication is accomplished through use of a radio, which is well known by those having ordinary skill in the art, connected to a radiating element, or antenna, also well know by those having ordinary skill in the art. An antenna is an impedance-matching device used to absorb or radiate electromagnetic waves. The function of the antenna is to “match” the impedance of the propagating medium, which is usually air or free space, to the source. Radio signals include voice communication channels, data link channels, and navigation signals.
Communication with those on the ground is most easily accomplished with radiating elements commonly called “monopoles” or “dipoles.” A dipole has two elements of equal size arranged in a shared axial alignment configuration with a small gap between the two elements. Each element of the dipole is fed with a charge 180 degrees out of phase from the other. In this manner, the elements will have opposite charges and common nulls. A monopole, in contrast, has only one element, but operates in conjunction with a ground plane, which mimics the missing second element. The physics of monopoles and dipoles are well known. Monopoles and dipoles, however, are efficient only for line-of-sight (LOS) communication. Obstructions such as mountains, or great distances, relative to the curve of the earth's surface, between the transmitter and receiver can prevent the reception of these signals. The relative positions of the transmitter and receiver, as well as the power output of the transmitter thus control whether the LOS signal will be received.
To overcome the effect of LOS obstacles, satellite communication (SATCOM) has been developed. Satellites are transceivers that orbit the Earth and can relay communications back and forth from the Earth's surface or to other satellites, allowing communication virtually anywhere in the world.
One of the characteristics of antenna transmission is “polarization,” which describes what physical plane the signal is being transmitted in. A dipole or monopole oriented in a vertical position (perpendicular to the earth's surface) radiates signals with a vertical polarization. For a second antenna to receive maximum signal strength, it too must have a vertical orientation. As the receiving antenna is rotated away from vertical, its maximum receive power diminishes until the antenna reaches a horizontal orientation (perpendicular to the transmit antenna), at which time the maximum receive power reaches zero.
Because satellites orbit the earth and transmit to receivers in multiple directions and orientations, single plane transmission is not efficient. Therefore, satellites transmit signals in a “circular” polarization. In this manner, the signal is transmitted in a continuous right-hand rotating orientation. A circularly polarized antenna has two dipoles arranged orthogonal to one another. The dipoles alternate “firing” with a positive charge rotating sequentially around the four individual elements and a negative charge on its axially oppositely aligned second element. When viewed on a three-dimensional time vs. polarization graph, the circularly polarized signal resembles a helix.
Due to the above-mentioned inherent loss in perpendicularly oriented linearly polarized transmitting and receiving antennas, a linearly polarized antenna will suffer from a 50% (3dB) signal loss when receiving satellite communication signals. Thus, a more efficient receiving means is desired.
“Man-Pack” radios are mobile radios designed to be carried or worn on a person. Currently Man-Pack radios are used by Military or Paramilitary soldiers in the field and used on the move or at halt. These radios employ a traditional monopole LOS antenna, which suffer from the above-mentioned inherent 3dB loss due to the polarization losses.
Portable SATCOM antennas, which are directional and circularly polarized, are available, however carrying two separate antennas is cumbersome. In addition, disconnection of the LOS antenna and connection of, and assembly or disassembly of a separate SATCOM antenna is usually burdensome to an excessive degree.
Accordingly, a need exists for a portable, lightweight, efficient, multiple band, multiple polarization, LOS/SATCOM antenna communication system in the form of a single unit that can easily be deployed in the field.